Getting right to the point--if you need a series for an 8-11ish year old kid who enjoys sci fi/fantasy, and who likes series fiction in which the character development and bigger picture is played out over the course of multiple books, I recommend the Gateway Series, by Cerberus Jones (Kane Miller, 2016 in the US, earlier in Australia), with a pretty high level of confidence.
The setting of the series is a mysterious old hotel, that happens to be located at a stop on the intergalactic expressway, more or less, and so many of the guests are aliens. Mostly they are harmless, but sometimes not....and no one on earth is supposed to find out about them. That being said, there are people on earth who know the secrets of the hotel.
When Amelia and her older brother James find out their parents are moving them to the strange old hotel, they have not idea that it has more mystery to it than the obvious "what were Mom and Dad thinking?' But as the various aliens passing through their new home stir up trouble and adventure, the kids, including Amelia's friend Charlie, whose mom works at the hotel, have no choice but to be involved, up to their necks in alien shenanigans....
Each book has its own entertaining adventure, and as the series progresses, character development progresses as well. As I said above, these are just at the right reading level for kids who don't want little kid chapter books anymore, but who are still young. The books are under 150 pages, with a generous font size, the dangers are dangerous without being too terribly scary, and Charlie and Amelia are relatable characters, who do their best to contribute helpful to every new crisis, with variable results....
The first four books of the series are
The Four-Fingered Man
The Warriors of Brin-Hask
The Midnight Mercenary
The Ancient Starship
disclaimer: review copies received from the publisher